The present invention relates to digitally producing patterns of copper wiring for printed circuit boards and, more particularly, to a process and materials for efficient imaging of etch resistant layers over the copper layer of a circuit board.
Printed circuit boards (PCBs) are built of an insulating substratexe2x80x94for instance, glass filled epoxy or polyimide filmxe2x80x94over which a thin conducting layer, usually of copper, in a pattern designed for any specific application, has been produced. The patterned conducting layer (also referred to as a printed circuit), is the means of carrying electrical voltages and currents between various electrical components, such as resistors, capacitors, integrated circuits and other electronic devices. The electrical components are soldered onto the copper printed circuit xe2x80x98wiresxe2x80x99 at a stage after the formation of the printed circuit.
According to conventional methods, the process of printing a specific circuit onto a PCB begins with a blank board, consisting of an insulating substrate and a copper layer (formed either by electrodeposition or as a rolled foil laminate. A photosensitive resist (photoresist) layer (whose function is described below) is spread over the copperxe2x80x94either by depositing it from solution or by providing it as a dry film and bonding it to the copper by pressure sensitive adhesive. A sheet of intermediate photographic film (known as a phototool) that contains a negative image of the circuit pattern is then superimposed on the photosensitive resist layer in close contact and this combination is exposed to a flood of ultra-violet (UV) light. Ultra-violet light transmitted through the transparent areas of the recording film (which correspond to prospective conducting portions of the circuit) causes the photosensitive layer under them to harden (i.e. polymerize). The unhardened material is then washed away with a suitable solvent, baring the underlying copper in the areas that were protected from the UV light. The board is then placed in an etch bath, where the bare copper is etched away, while portions of the copper layer that lie under the remaining resist layer are protected from the etching action. Finally, the remaining (hardened) photoresist is removed with a suitable solvent.
Since nowadays the circuit pattern (i.e. wiring configuration) is produced digitally on a computer, the aforementioned phototool (intermediate film) is usually produced by laser recording the image on the film from the digital data, using an imagesetter. The process of thus producing the phototool involves several steps, including chemical processing of the film. It is, therefore, costly, in terms of both labor and materials. It would thus be economically advantageous if the image data could be transferred directly from the computer onto the circuit board, circumventing the entire process of producing the phototool. The elimination of the intermediate film, with its chemical processing, would also have environmental advantages.
Various attempts have been made to digitally print PCBs directly. U.S. Pat. No. 5,270,368 describes a method for image-wise applying an etch-resistant material onto the copper layer, using ink-jet techniques. It discloses resist compositions suitable for jetting, which consist of a mixture of at least two acrylates plus a photoinitiator plus an organic carrier (solvent). The viscosity of the composition is from about 1 to 10 centipoise. Upon jetting the material onto the copper layer, the solvent evaporates and the resist is set; it is then hardened by exposing it to a flood of UV light for curing. The main disadvantage of the jetting method is its limited spatial resolution; the wiring of modern PCBs contains features measured in tens of micronsxe2x80x94which is beyond the practical capabilities of ink-jet techniques because of inaccuracy of dot placement and difficulty in producing very small dots.
U.S. Pat. No. 4,717,639 describes a method for image-wise applying an etch-resistant material onto the copper layer, using electrophotographic techniques, akin to xerography. It discloses a specially formulated toner powder, which is printed onto the copper and which, following the etching stage, is removed with strong alkali. This method, as well, places limitations on the achievable resolution because of the limitation on the smallest toner particles to be used.
Rather than directly printing a resist pattern onto the copper, an alternative approach, which inherently yields much higher resolution, is to coat the copper layer with a photoresist (as in the conventional process) and to form thereon a mask, which is opaque to the UV light that subsequently floods the photoresist to harden it. Methods and processes for forming a UV-opaque mask by digital writing are known in the realm of direct digital production of printing forms for printing presses. U.S. Pat. No. 4,132,168 describes a laser ablated masking for an offset-lithographic plate. The plate consists of a conventional aluminum substrate, coated with a conventional UV sensitive layer and over thatxe2x80x94a layer opaque to UV (which is deposited either from vacuum or from solvent). The latter is selectively removed by ablation, using a laser beam. The non-removed opaque material then acts as a mask for the UV exposure of the underlying sensitive layer. The mask is then removed and subsequently the plate is processed to remove the non-exposed sensitive layer, thus forming a lithographic image. The process disclosed in the ""168 patent is not suitable for PCB production, since the photosensitive layer cannot act as an etch-resist and, moreover, generally has a thickness of the order of 1 micron or less.
U.S. Pat. No. 5,607,814 describes a masking method for producing a flexographic printing form (or plate) that uses a transfer process. The plate consists of a substrate and a thick photosensitive layer, which contains photoinitiators. A cover sheet, coated with at least one infrared (IR) sensitive layer, is placed over the plate and exposed to an IR image. The IR exposed areas transfer onto the UV sensitive layer and subsequently act as a mask while the layer is flooded by UV light. The unmasked areas of the layer polymerise (to eventually become the printing surfaces) and the remaining (unpolymerised) material, as well as the mask, is subsequently removed. The process disclosed in the ""814 patent is, again, not suitable for PCB production, since the photosensitive layer is much too thick and consists of elastomeric material, which is not inherently etch resistant. Moreover, the photosensitive elastomeric layer, which serves as the printing form, is necessarily designed to be sturdy and durable, whereas the photosensitive resist layer of a PCB must be easily removable after the etching process.
U.S. Pat. No. 5,262,275 also describes a UV masking method for producing a flexographic printing plate, but here the mask is formed of a layer of infrared ablatable material. The patent discloses a barrier layer between the IR and the UV layer, to prevent migration of materials from the UV layer into the IR layer and to prevent oxygen inhibition of the UV curing process. This method is not suitable for PCB production, for the reason given above, with respect to the ""168 and ""814 patents, and because it uses a strong solvent, such as methylene chloride, to remove the mask, which solvent may attack the UV sensitive layer if it were as thin as required in the PCB etching process.
There is thus a widely recognized need for, and it would be highly advantageous to have, a method and a process for digitally forming a UV-opaque mask directly over an etch-resistant photosensitive layer, usable in the course of PCB production, which permit each layer to function without chemical interaction therebetween.
The present invention successfully addresses the shortcomings of the presently known configurations by providing a blank printed circuit board (PCB) with only two coatings over the metal layer, which have improved properties that enable writing the wiring pattern on such a PCB with an image of infra-red (IR) radiation, by image-wise ablating a mask coating, subsequently irradiating an underlying photosensitive resist coating by ultra-violet (UV) radiation, modulated by the mask, and etching the metal while protected by the patterned resist.
The present invention discloses a novel composition of both the resist coating and the mask coating which simplify the process and make it more consistent and reliable.
More specifically, the blank PCB of the present invention consists of a conventional insulating substrate, such as glass-filled epoxy, laminated to a copper foil, on top of which is a resist layer, which is coated with a mask layer. The resist layer consists of three components: (a) a group of oligomers and monomers that polymerize under UV irradiation, in the presence of photoinitiators, (b) photoinitiators and synergists that generate free radicals for cross linking the resins of component xe2x80x98axe2x80x99, and (c) binder resins. The mixture of the three components is deposited on the copper out of an organic-solvent solution, to form a 12 microns thick film. Component xe2x80x98axe2x80x99 forms 35%-75%, component xe2x80x98bxe2x80x99xe2x80x94less than 10% and component xe2x80x98cxe2x80x99xe2x80x9410%-50% of the film, by weight. The mask layer consists primarily of carbon blackxe2x80x94up to 100%, which is deposited out of an aqueous suspension, to form a coating whose thickness is in the order of 1 micron.
The resist layer has the basic property of becoming cured when component xe2x80x98axe2x80x99 polymerizes, making it insoluble in water (or dilute aqueous alkali solution) and resistant to metal etching agents, used for etching the underlying metal foil. The binder resins (component xe2x80x98cxe2x80x99) and their proportion are chosen to be soluble both in water (or dilute aqueous alkali solution) and in organic solvents and to have the essential properties of (1) adsorbing to the surface of the resist layer some of the carbon black from its aqueous suspension and thus rendering the deposited mask layer rub resistant and (2) while remaining water soluble in the uncured state, allowing the polymerised component xe2x80x98axe2x80x99, in the cured state, to remain water resistant. Aqueous solubility in the uncured state preferably extends also to etching solutions, so that the washing and etching can proceed as a combined process. The adsorption property (1) is of such nature that it does not interfere with ablation of the mask layer (as described herebelow).
Carbon black has been chosen for the mask layer because (1) it absorbs IR well and (2) it greatly attenuates UV radiation, by absorption, and thus masks it from the underlying resist layer during the curing process. The layer""s thickness has been chosen to optimize both these functions. Aqueous suspension is specified, because an organic solvent may cause some carbon black to diffuse into the resist layer, thus evading ablation, and, on the other hand, it is not necessary for rendering the coating rub resistant, because of the binder resin in the resist layer, as explained hereabove. For the same reason, the mask layer itself also does not need to contain binder resins, thus allowing a greater density of carbon black and hencexe2x80x94greater sensitivity to IR.
During production of a PCB with a specific wiring pattern, the blank PCB, which is composed as described hereabove, is first irradiated by scanning beams of high-power-density IR radiation, e.g. from diode lasers, switched on/off according to digitally stored data representing the desired wiring pattern. Irradiated portions of the mask are ablated, leaving underlying portions of the resist exposed. These are subsequently irradiated by a UV flood (while masked portions remain protected) and are consequently cured. The PCB is then washed with water or alkaline solution, which removes all remaining mask portions and uncured resist, baring the underlying copper portions. Thereafter (or concurrently) the PCB is immersed in an etching solution, which etches away the bared copper. Finally the PCB is wiped with an organic solution, to remove all remaining (cured) portions of the resist, leaving the underlying patterned copper.
According to the present invention there is provided a blank printed circuit board (PCB), for creating a circuit pattern thereon by direct imaging with infra-red radiation, comprising in sequence an insulating substrate, a metal layer, a resist layer and a mask layer, wherein:
The resist layer has been deposited from an organic solution, has a thickness of between 3 microns and 30 microns and includes three components
a first component, constituting 35% to 75% of the layer by weight and including acrylic monomers and oligomers, capable of polymerising when and where irradiated by ultra-violet radiation in the presence of photoinitiators, such polymerisation constituting curing of respective portions of the resist layer,
a second component, constituting up to 10% of the layer by weight and including photoinitiators and synergists, and
a third component, constituting 10% to 50% of the layer by weight and including binder resins that are soluble in water or in dilute aqueous alkali solvents, as well as in non aqueous solvents; and
The mask layer has been deposited from an aqueous suspension, has a thickness of between 0.3 microns and 6 microns, and includes between 80% and 100% by weight of carbon black and not more than 10% by weight of binder resins.
According to further features in preferred embodiments of the invention described below, the mask layer is ablatable by infra-red irradiation and directly adjoins the resist layer, uncured portions of the resist layer are entirely soluble in water or in dilute aqueous alkali solvents, the resist layer has the further property of adsorbing carbon black from aqueous suspensions so as to render the mask layer rub resistant, and binder resins have the further property of not preventing cured portions of the resist layer from being resistant to aqueous alkali solvents.
According to still further features in the described preferred embodiments, the resist layer, where not cured, is also soluble in metal etching agents, has the further properties of thermally insulating between the metal foil and the mask layer and of not allowing diffusion thereto of carbon black from the mask layer in quantities that would interfere with the polymerisation.
According to another aspect of the invention there is provided a method for fabricating a blank PCB that is amenable to creating a circuit pattern thereon by direct imaging with infra-red radiation, such as specified hereabove.
According to yet another aspect of the invention there is provided a method for fabricating a printed circuit board (PCB) with a wiring pattern formed thereon according to digitally provided image data, comprising:
(a) Providing a blank PCB that includes four layers, the four layers being, in sequence, an insulating substrate, a metal foil, a resist layer and a mask layer, wherein the resist (IR) radiation;
(b) Exposing the mask layer to IR radiation whose intensity varies over the upper surface of the mask layer between a low value and a high value according to the image data, the high value being such as to cause ablation of the mask layer;
(c) Exposing the resist layer to a flood of UV radiation, such that causes the resist layer to cure under all areas that correspond to portions of the mask layer that have been ablated, whereby all portions of the resist layer that lie under unablated portions of the mask layer remain uncured;
(d) Washing away all unablated portions of the mask layer and all uncured portions of the resist layer, so as to bare the upper surface of the metal layer in essentially all areas that correspond to uncured portions of the resist layer;
(e) Subjecting the PCB to a metal etching agent such that etches the metal layer away under all areas that have been bared in step d, whereby essentially all portions of the metal layer that lie under cured portions of the resist layer remain intact;